Personally if it were me I would do the 6BR if you wanted a small 6mm. There is only about a grain or so difference in capacity and about 50fps difference in performance with the edge going to the BR. Also, the other nice things about the 6BR is the fact that there is multiple quality brass options out there and it fits a standard bolt face. The 6ARC is Hornady re-inventing the wheel...again and putting a cartridge that already existed in a shiny red package to sell to the shovel headed masses just like they did with the 6/6.5 Creedmoor, and 6.5PRC.